1. Field of the Invention
The present invention relates to a hydraulic system for a construction machine.
2. Description of the Related Art
In a conventional hydraulic circuit of a hydraulic excavator there has been a problem of interference between pressure oil fed to a traveling motor and pressure oil fed to a working actuator in case of performing both travel using the traveling motor and work using the working actuator. In this case, it is difficult to maintain the operation speed of the traveling motor, i.e., the traveling speed of the hydraulic excavator, stably at a desired relatively low speed.
It is an object of the present invention to provide a hydraulic system for a construction machine capable of preventing the occurrence of interference between pressure oil fed to traveling motors and pressure oil fed to working actuators and smoothly performing a work by operation of the working actuators under travel at a stable speed particularly in case of carrying out both travel and work by the working actuators at a time.
A hydraulic system for a construction machine according to the present invention, comprising: a first traveling motor and a second traveling motor adapted to actuate a pair of travel devices; actuators adapted to actuate working attachments including a boom and an arm; a first hydraulic pump and a second hydraulic pump adapted to supply pressure oil for actuating said first and second traveling motors and said actuators; a first traveling control valve and a second traveling control valve adapted to control amount of pressure oil to be supplied to said first and second traveling motors in accordance with operation of operating means for the first and second traveling motors; working control valves provided correspondingly to said actuators, said working control valves being classified into a first group including said first traveling control valve and a second group including said second traveling control valve, bleed-off passages in all the control valves belonging to said first group being mutually communicated in series as a first center bypass passage toward an oil tank when all the control valves are in their neutral positions, and bleed-off passages in all the control valves belonging to said second group being mutually communicated in series as a second center bypass passage toward an oil tank when all the control valves are in their neutral positions; a straight-travel valve adapted to switch each flowing direction of pressure oil discharged from said first and second hydraulic pumps, said straight-travel valve supplying pressure oil discharged from said first and second hydraulic pumps to said first and second bypass passages respectively when all of said traveling motors and said actuators are not in operation, while in a simultaneous operation mode in which the traveling motor and the actuator associated with the traveling control valve and the working control valve belonging to one of said first and second groups are operated simultaneously, supplying pressure oil discharged from one of said first and second hydraulic pumps to both said first and second traveling control valves and further supplying pressure oil discharged from the other hydraulic pump to the working control valve; and a cut-off valve and an opening valve provided on a downstream side of each of the bleed-off passages in said traveling control valves, said cut-off valve cutting off the center bypass passage located between the traveling control valve and the working control valve associated with the traveling motor and the actuator which are in operation in said simultaneous operation mode of the traveling motor and the actuator being operated simultaneously, said opening valve causing a downstream side of the bleed-off valve in said traveling control valve to be opened to the oil tank.
In this case, the hydraulic system is capable of preventing the occurrence of interference between pressure oil fed to traveling motors and pressure oil fed to working actuators and smoothly performing a work by operation of the working actuators under traveling at a stable speed particularly while carrying out both traveling and work by the working actuators at a time.